Papers by Keyword: Age Hardening

Abstract: This research is aimed to study effect of holding time and temperature in solution treatment and ageing on the hardness of the Aluminum A356 reinforced with silicon carbide particles. Alumimium matrix coposite was particulated with 15 microns-SiC with the content of 15 percent by weight. A356/SiC composites was prepared by the stirr casting. The stirrer was continuously stired at a speed of 300 rpm during acooling temperature to semi-solid state of 610 °C. During stirring the SiC powder was slowly added into the melt and continuously stirred for another 10 min. Then poured into the mold at the pouring temperature of 680°C. A356/SiC composites then subjected to a solid solution treatment with temperature of 540 °C for 1 hr. quenched with water and age hardening temperature of 120 and 135 °C for 3, 6, 12 and 18 hr. The result showed that, in as cast conditions, the composite with 15 microns silicon carbide, the 15 wt%SiC specimen exhibited the average hardness of 64.33 HB. After solution treated, the hardness of samples decreased. the specimen exhibited the average hardness of 53.83 HB. After ageing, the hardness of samples increased. the specimen ageing 120 °C 6 hr. exhibited the lowest average hardness of 51.45 HB while the specimen ageing 135 °C 18 hr. exhibited the highest average hardness of 73.54 HB.

Abstract: In this study, effect of die pressure and injection speed on hardness and microstructure in rheo-die casting of A356-SiC composite was investigated. The master A356-SiC composite was first produced by the mechanically stirred casting. SiC-15 micron particle was added 15 wt% in the molten A356. Master composites were then remelted at 610-615 °C. Then the slurry was transferred to a ladle and injected into the die. The Injection speeds were 3 and 4 m/s and die pressures were 11 and 12 MPa. Slurry was injected into a rod of 16×15.6×205 mm. Samples were then subjected to T6 treatment: solution treated at 540 °C for 1 h, water quenched then aged at 135 °C for 12 h. The result showed that hardness increased with increasing both speed and die pressure. At speed of 3 m/s and die pressure of 11 MPa which was an injection condition recommended for the molten A356, rheo-casted sample exhibited uneven filling at the end of the rod. When both speed and die pressure increased, samples were successfully and evenly filled the die cavities. Brinell hardness tests were performed in both as-rheo casted and T6 conditions. In as-rheo casted condition, the maximum hardness value of 82.16 HB obtained from a sample rheo-casted with speed of 4 m/s and die pressure of 12 MPa. A microstructure in cross section area of a rod revealed a uniform distribution of SiC particles in the A356 matrix. After T6, hardness value of composites increased approximately 15.6%.

Abstract: The present study deals with the effect of pre-deformation technique of AA2519 alloy (Al–5.64Cu–0.33Mn–0.23Mg–0.15Zr–0.11Ti–0.09V–0.08Fe–0.01Si (wt. %)) under T8 tempers, on condition that intermediate strains are equal. After undergoing T87 by pre-stretching and peak ageing the alloy demonstrates the yield stress, ultimate tensile strength and elongation to failure of 425.4±2.4 MPa, 475±2.4 MPa, 12.1±0.4%, respectively. The 7% pre-straining by rolling leads to-5% decrease in yield stress, practically the same ultimate tensile strength and-20% decrease in ductility. This effect can be ascribed to more homogeneous distribution of dislocations which act as heterogeneous nucleation sites for the θ′-phase precipitation. In addition to precipitates of the Al–Cu family (θ′′ and θ′), Ω-phase plates on {111}_α habit plane was observed. The effect of pre-straining prior to ageing on the precipitation behavior and its relation with mechanical properties of the AA2519 is considered.

Abstract: This paper explains the type of heat treatments used to improve the bulk hardness and tensile properties of Tungsten Inert Gas (TIG) welded Al6061-SiC reinforced composites. Composites with 8, 10 and 12 wt% of SiC (25 μm) are TIG welded using ER5356 filler material and the hardness, tensile property and microstructural analysis in as weld condition is carried out. Age hardening heat treatment is performed and peak hardness values are noted at different aging temperatures and subjected to characterization. Al6061-12 wt% SiC shows reduction in tensile strength in comparison with the other two in spite of improvement in hardness. Since hardness is more, there arise ambiguity in knowing the actual cause of decrease in the tensile strength. Optical micrographs of the samples are taken and identified the presence of dendritic segregation as the actual cause for the (UTS) reduction. To remove dendritic segregation, all the as weld samples are subjected to 5h homogenizing at 500°C, age hardened and characterized. The result showed substantial hike in UTS with substantial improvement in hardness, homogeneous distribution of hardness over the cross-section in the weld bead. The depth and distribution of hardness and bulk property improvement are observed by the combined effect of homogenizing and low temperature age hardening.

Abstract: This report presents a comparative assessment of the suitability of two existing physical models for predicting yield strength of artificially aged AA6063 Al-alloy. One model is based on the modified Orowan mechanism of dislocation by-pass for non-shearable rod shaped precipitates rejuvenated by Zhu and Stark, and the other model is based on classical dislocation-particle interaction that incorporates both cutting and by-pass mechanisms for spherical shaped precipitates developed by Deschamps and Brechet. Using these models, simulation of yield strength values have been performed considering nucleation-growth as well as nucleation-growth-coarsening of precipitates during the entire period of ageing. Comparison of experimental and simulated results reveals that the model by Deschamps and Brechet predicts yield strength more accurately when nucleation-growth-coarsening of precipitates are incorporated.

Abstract: Traditionally, Mg-Gd alloys have been strengthened by dispersed precipitates. Several reports are available about Sc addition to Mg alloys for improving a creep resistance. In this research, aging behavior of Mg-Gd, Mg-Sc and Mg-Gd-Sc alloys including the same amount of solute elements were investigated to understand the effect of Sc on microstructures and mechanical properties during aging. Hardness measurement revealed that Sc addition delayed to form precipitate. Close inspection of TEM micrographs, β” phase formed at an initial stage of aging and β’ phase was observed at a peak-aged stage in Mg-Gd and Mg-Gd-Sc alloys. In Mg-Sc alloy, there is no evidence of precipitate formation during aging at 473K.

Abstract: Al-Li alloys have higher mechanical properties and more lightweight than other conventional aluminum alloys. Therefore , it is focused as a good material for weight reduction of industrial fields. However, since the Al-Li alloy are highly active and hard to cast, there has been limited research on casting. In this study, age-hardening behavior of Al-2.5mass%Li alloys cast into sand and metal mold were investigated. All alloys cast into Y-block shape sand mold, and then artificial aged after solution treated at 743K for 36ks. Because of difference in quantity of precipitation by metastable δ’(Al₃Li) phase, peak hardness of metal mold casting is higher than that of sand molds castings.

Abstract: The manufacturing of AA6xxx car body panels typically consists of rolling, ageing and forming processes. Thus, multiple simulation tools can be coupled to set up a through-process modelling (TPM) framework for predicting the evolution of microstructure and the final mechanical properties of these products. In order to realize such a TPM concept, various industrial processing phenomena were studied and modelled in the open innovation research cluster “Advanced Metals and Processes” (AMAP^♯). This work focuses on the age hardening behavior which takes place during the industrial paint bake process. To reflect the microstructure evolution of this processing step, a multi-component precipitation model is developed. So far, the influences of thermomechanical processes, i.e. annealing temperature on the kinetics of Mg_xSi_y precipitates during artificial aging were implemented. The precipitation model was linked to a yield strength model in order to simulate the evolution of mechanical properties within the TPM framework. For validation, the evolution of microstructure and mechanical properties of an AA6016 alloy during artificial ageing was investigated via transmission electron microscopy (TEM) and tensile testing. The simulation results are in agreement with experimental observations.

Abstract: Quenching is a critical step during the strengthening age hardening of Aluminium alloys. To obtain optimal technological results, parts should be quenched with the upper critical cooling rate. The precipitation behaviour of Al alloys during cooling from solution annealing and thereby the critical cooling rates are typically investigated by in-situ measurements with differential scanning calorimetry (DSC). Conventional DSCs are limited at cooling rates below 10 Ks^-1. Unfortunately, medium to high strength Al alloys typically have critical cooling rates between 10 and some 100 Ks^-1. Recently it was shown that dilatometry is generally able for in-situ detection of precipitation in Al alloys. Dilatometry allows controlled cooling up to some 100 Ks^-1 and therefore covers the cooling rate range relevant. In this work, we aim to show up and discuss possibilities and limitations of dilatometric detection of quench induced precipitates in 2xxx, and 7xxx Al alloys. The basic method will be presented and results will be compared with DSC work.

Abstract: Precipitation hardening has been used before as one of the most effective strengthening methods for many metallic alloys. However, this method has not been studied completely in magnesium alloys, and the numbers of precipitation hardenable wrought Mg alloys are still very limited compared to aluminum alloys and steels. The age hardening responses of Mg-Al-Sn alloys in cast-homogenized condition were investigated by isothermal aging at 200°C for prolonged time. It was found that hardness can be improved significantly for the alloy with higher amounts of tin. The improvement in hardness was reasoned by the formation of precipitates. The shapes and morphology of the precipitates were different depending on the orientations of the grains. The precipitates were characterized by scanning electron microscope.